Unit 09b : Advanced Hydrogeology

Isotopic Processes

Isotopes

• Atoms of the same element that differ in terms of their mass are called isotopes.

• Hydrogen has three isotopes 1H, 2H, 3H• 1H and 2H (deuterium) are stable isotopes.• 3H (tritium) is radiogenic and decays (with a half life

of about 12.3 years.• Relative isotopic concentrations change in

processes where mass fractionation is important such as evaporation, condensation and water-rock interactions.

Radioactive Decay

• Radioactive decay involves emission of an alpha particle (4He), a beta particle (0e), or gamma radiation produced when atoms in an excited state revert to their ground state

Alpha Decay 232Th > 228Ra + 4He

Beta Decay 228Ra > 228Ac + 0e

Gamma Emission 236U* > 236U +

Isotopic Decay• Radioactive decay is an irreversible reaction.

• Decay follows a first order rate law:

dN/dt = -kN

• Solving for N gives:

N(t) = Noexp(-kt)

• The time when N(t) = 1/2 No is called the half-life.

1/2 = exp(-kt1/2)

ln(1/2) = - kt1/2

-ln(2) = -kt1/2

t1/2 = ln(2) / k = 0.693 / k

Radioisotopes

• Radioisopes are important for two reasons:– they are contaminants that represent a

radiation hazard to living tissue.– They provide a means of age-dating

groundwaters

• The most important radioisotopes used in age-dating waters are 3H, 14C,

32Si and 36Cl.

Half-Lives3H 12.3 y

32Si 100 y14C 5300 y

36Cl 310,000 yTo obtain “ages”, concentrations of isotopes must be measured in mass spectrometers. The analysis becomes difficult after about 5 x the half life. So tritium is good to about 60 years, 32Si to about 500 years, radiocarbon to about 26,000 years and 36Cl to around 1.5 My.

Isotopic Abundance• The isotopes of H, O, C and S are ubiquitous in natural

groundwaters and are useful in studying chemical processes.

Oxygen16O 99.76217O 0.03718O 0.100

Hydrogen1H 99.9842H 0.0153H* ~10-16

Carbon12C 99.8913C 1.1114C* ~10-10

Sulphur32S 95.0233S 0.7534S 4.2136S 0.02

• Relative abundance data show that one isotope dominates the rest in all cases.

• Radioisotopes (3H and 14C) have very low abundance

Delta Values

• Isotopic ratios are reported as positive or negative deviations from a standard:

= Rsample- Rstandard x 1000

Rstandard

The units for are parts per thousand or permil.• For example, the oxygen isotopic ratio 18O/16O :

18O = (18O/16O )sample- (18O/16O )standard x 1000

(18O/16O )standard

A value of 18O of -20%o means that a sample is depleted in 18O by 2% relative to the standard.

Stable Isotopes in Water

• Water contains hydrogen and oxygen both of which have two or more stable isotopes.

• 2H and 18O compositions are usually measured with respect to the Standard Mean Ocean Water (SMOW) standard.

• SMOW is a particularly appropriate standard for groundwater studies because precipitation that enters the groundwater system originates from evaporation of ocean water.

Fractionation• Water vapour in equilibrium with water is typically

depleted in heavy isotopes by 80%o 2H and 10%o 18O so for water vapour in equilibrium with ocean water the isotopic ratios are 2H = -80%o and 18O = -10%o

• The depletion of heavy isotopes in water as a result of evaporation is an example of a fractionation process.

• Water vapour in air masses is typically not in equilibrium with ocean water. Near the equator vapours are slightly depleted and the depletion increases with latitude (mainly because of temperature differences).

Latitude Effect

• The “latitude effect” is related to progressive temperature-controlled removal of heavy isotopes from the vapour during precipitation.

• At lower temperatures, the fractionation is more pronounced but all rainwaters are systematically depleted relative to SMOW.

• When isotopic compositions of rainwaters from around the world are plotted, they lie along a straight line known as the meteoric water line:

2H = 818O + 10%o

Meteoric Water Line

-120

-100

-80

-60

-40

-20

0

20

-14 -12 -10 -8 -6 -4 -2 0

18O permil

2H

per

mil

MWL

Meteoric Water Line• Key to interpretation of 2H and 18O• Water falling on the line is assumed have

originated from atmosphere.• Deviations from the line are caused by other

processes– Water/rock interactions (O only)

– H2S exchange reactions (H only)

– Open surface evaporation– Condensation

-120

-100

-80

-60

-40

-20

0

20

-14 -12 -10 -8 -6 -4 -2 0

18O permil

2H

per

mil

CO2-exchange

Isotopic Reaction Paths

MWL

H2S-exchange

Water / Rock interaction

Evaporation

Condensation

Silicate Hydration

Age Dating

• There are two approaches:– Direct

• interpret concentration distribution of naturally occurring radioisotopes (3H, 14C, 36Cl)

– Indirect• interpret changes of stable isotopes () or

organic tracers CFC’s (chlorofluorocarbons) in recharge

Direct Method

• The residence time of a radioisotope in the system (or the age of the recharge water) is given by:

t = t1/2 ln (Ao/Aobs) / ln(2)

where Ao is the presumed initial activity of the isotope and Aobs is the observed activity.

Tritium

• 1 TU = one 3H atom per 1018 atoms 1H• Typical natural tritium levels at ~ 20 TU• Bomb tritium in N.America peaked at >2000 TU in

1963

• Problem with 3H for dating is that Ao has to be estimated from historic records– 30 TU water in year 2001– could be 1952 water (500 TU) after 4 half lives 500/16 30– could be 1976 water (75 TU) after 2 half lives 75/4 30

• Main use is to identify pre-test (pre-1952) waters

Radiocarbon

• Measurements are reported as percent modern carbon-14 (pmc). So current source is 100 pmc.

• 14C in groundwater comes from solution of CO2(g) in the soil zone.

• Method assumes that carbonate in the saturated zone carries 14C and that no additional C is added to the system.

• Age dating is valid if the only process removing 14C is radioactive decay.

• This is not generally the case.

Processes affecting 14C

• Dissolution of carbonate minerals may add “dead carbon” to the system.

• Oxidation of organic carbon, sulphate reduction and methanogenesis may add “dead carbon”.

• Precipitation of carbonates may remove radiocarbon.

• Isotopic exchange with carbonates may remove radiocarbon.

Correction Procedures• Procedures have been devised to correct 14C activities for the

various processes adding “dead carbon” and removing 14C• These procedures include:

– empirical “average” estimation of “dead carbon”– accounting for calcite solution only as a source of “dead

carbon”– using 13C to estimate “dead carbon” addition.– complex geochemical modelling procedures accounting for

all processes• Uncertainties make 14C dating at best a semi-quantitative

tool.

Chlorine-36• 36Cl is a potentially useful tool for dating waters up to 2

million years old.• Few natural processes add “dead chlorine” so problems

are much reduced by comparison with 14C.• Source is atmospheric and believed to be constant (like

natural 3H and 14C)• Typical 36Cl values for meteoric waters lie in the range

100 to 500 x 10-15 for 36Cl/34Cl ratios.• Atmospheric bomb testing elevated 36Cl levels by 2 or 3

orders of magnitude so the bomb-pulse can be used in the same way as 3H.

Indirect Method

• Method relies on interpreting systematic changes in stable isotopes along groundwater flow paths.

• Unlike radioisotopes, the time marker is provided by an interpreted event that changed the tracer in a systematic way.

• Shifts in stable isotope ratios are apparent in groundwater profiles.

Climate Change• Shallow groundwaters show -9 to -10%o 18O• Deeper waters give -14 to -17%o 18O• The shift is interpreted as gradual climatic warming

following deglaciation and has been “calibrated” using radiocarbon.

• The more depleted water is indicative of older infiltration generated at a time when the climate was colder.

• Method provides a useful time scale in low K materials where groundwater moves slowly.